The prevalence of overweight and obesity is increasing worldwide. 1 A comparison of data from 1976–802 with that from 1999–2000 shows that the prevalence of overweight (defined as body mass index, BMI, of 25–29.9 kg/m2) increased from 46% to 64.5%, and the prevalence of obesity (BMI 530 kg/m2) doubled to 30.5%. The epidemic of obesity is not just isolated to the US, but is worldwide, 3, 4 including less affluent countries. 4 Obesity and overweight have many causes, including genetic, metabolic, behavioural and environmental. The rapid increase in prevalence suggests that behavioural and environmental influences predominate, rather than biological changes. We summarize data from many studies evaluating the impact of obesity on mortality and morbidity, discuss some controversies and provide practical guidelines for managing obese patients.